The invention relates generally to combustors, and more particularly to a coanda pilot nozzle for low emission combustion devices.
A gas turbine employed in, for example, a gas turbine plant or a combined cycle plant is operated to achieve higher operational efficiency under higher temperature and higher pressure conditions, and this tends to increase emissions (NOx) in an exhaust gas stream. Although various factors for generation of NOx are known, the dominant one is flame temperature.
There are some conventional techniques for reducing NOx in an exhaust gas stream. One conventionally adopted method involves injection of steam or water into the high-temperature combustion area in a combustor for reducing the flame temperature during the combustion. Although this method is easy to carry out, it suffers from a problem that a large amount of steam or water is required. As a result, plant efficiency may be reduced. Moreover, injection of a large amount of steam or water into the combustor increases combustion vibrations and partial combustion products.
Taking the above defects into consideration, a dry type premixed multi-stage lean combustion method has been developed, in which fuel and combustion air are premixed and burned under lean fuel conditions. In this premixed multi-stage lean combustion technique, a flame structure which provides a diffusion combustion flame is used to ensure stable combustion over a wide fuel-air ratio range. While improving the dynamics, and operability of the combustor, such a technique tends to generate higher NOx due to local hot spots in the flame. The NOx emission levels become larger, since NOx emissions are directly proportional to the flame temperature.
Accordingly there is a need for a system that is employed in gas turbines that achieves reduced NOx emissions from the combustor without compromising the dynamics and operability of the combustor.